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Nvidia released whitepapers that shed new light on its upcoming Tegra processor, codenamed Kal-El this week, including information on the SoC’s unusual CPU configuration. The new chip, as it turns out, isn’t a conventional quad-core design. It contains an additional fifth ‘Companion Core’ that’s invisible to the operating system and meant to help reduce standby battery life and reduce power consumption. It’s more accurate to refer to Kal-El as a 1+4 system or, if you prefer an arbitrary written term, a Uni-Quad CPU.

TSMC offers three basic types of 40nm silicon: 40G (General), 40LP (Low Power), and 40LPG. Components built using 40G are designed to hit high frequencies without requiring large amounts of additional voltage to do so. Idle power consumption is less of a concern here, as it’s assumed that the device will spend much of its time in high-performance mode. Desktop discrete GPUs are a good example of hardware typically manufactured on “G” silicon.

LP silicon is designed to minimize leakage current and runs at a lower speed. Chips built on LP silicon don’t scale particularly well; the optimizations that minimize leakage currents at low clock speeds mean that it takes more voltage to push the processor past a certain point. The graph below demonstrates how two different types of silicon produce two different power/performance curves.

Kal-El’s design is Nvidia’s attempt to eat its cake and have it, too. The chip uses TSMC’s LPG manufacturing, which allows an area of “G” transistors in an otherwise-LP design. The Companion core is built on LP silicon and runs at 500MHz or less. All of the system’s standby needs and background processing are handled by the Companion core. According to Nvidia, this combined approach allows the company to hit the performance of CPU A and the low power consumption of CPU B. Using two different types of silicon improves the chip’s power consumption but raises a different issue — how does the OS know which cores can be scaled to which frequencies?

The answer is, it doesn’t — and it doesn’t need to. Nvidia has designed the switchover from Companion core to primary CPU to be both seamless and transparent to the operating system. The system powers up (or down) the number of cores at any given moment depending on which workloads it’s being tasked with handling. Nvidia calls this approach vSMP (Variable Symmetric Multiprocessing). The difference between vSMP and SMP is that the former is designed to efficiently juggle workloads across heterogeneous processors with different levels of performance.

With apologies to Valve. We couldn't help it.

The Companion Cube core and the four cores around it. All five cores are never active simultaneously

The Companion core is effectively camouflaged from the OS; Android (or Windows 8) doesn’t recognize the fifth core as existing separately from the other four. One of the surprises of Kal-El is that the chip’s clocking scheme is relatively simple. All of the SoC’s active cores run at the same clock speed, period. There’s no asynchronous clocking; Nvidia claims that implementing the feature would “increase BOM (Bill of Materials) cost and power consumption” due to the need for separate voltage rails for each core.”

Nvidia’s decision not to implement asynchronous clocking likely has more to do with wanting to keep its own design simple than any flaws in that approach to power conservation, the company’s protestations notwithstanding. Team Green’s description of Kal-El’s approach to power conservation suggests that the SoC tends to prefer spinning up more cores as opposed to cranking up clock speeds.

All active cores run at the same speed

There’s no general answer to the question of whether four slower cores are more power efficient than two fast ones; the answer depends on the hardware’s frequency scaling, the type of work being performed, and the number of background tasks already occupying CPU time. Kal-El’s ability to maximize potential performance per watt will depend on how accurately Nvidia has modeled various scenarios.

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